1. Technical Field of the Invention
The present invention generally relates to Ethernet networks. More particularly, and not by way of any limitation, the present invention is directed to a system and method for detecting loops in a customer-provider bridge domain of an Ethernet network.
2. Description of Related Art
The link between the end user and the public network, essential key to the delivery of broadband applications to residential and business subscribers, is known by many names, e.g., first mile, last mile, local loop, metro access, subscriber access network, etc., and is implemented using a variety of different transport technologies and protocols over diverse physical connections. For instance, today most users connect to the public network with Digital Subscriber Line (DSL), Integrated Services Digital Network (ISDN), cable TV, T1/E1 or T3/E3 lines, using Synchronous Optical Network and its companion Synchronous Digital Hierarchy (SONET/SDH), Frame Relay and Asynchronous Transfer Mode (ATM). Regardless of the nomenclature or the actual implementation, all access networks require operations, administration and maintenance (OAM) support features to ensure the maintainability and uptime required to provide broadband services.
Current first/last mile solutions have significant shortcomings from the customer's perspective, ranging from performance bottlenecks, fixed bandwidth provisioning, limited scalability, lack of flexibility and provisioning complexity to end-to-end quality of service (QoS) issues and a high cost structure. The use of robust, simple Ethernet technology in the first mile promises to revolutionize the access network as it did the enterprise network. Ethernet is a local area network (LAN) transport technology that is used ubiquitously in the home and in business to communicate between computers and networks. As an access technology, Ethernet offers three significant advantages over legacy first mile technologies: (i) future-proof transport for data, video and voice applications; (ii) cost-effective infrastructure for data services; and (iii) simple, globally accepted standard that will ensure interoperability.
As is well-known, various standards are being specified to enhance service delivery technologies, which allow provisioning of Virtual LANs (VLANs) on top of a Layer-2 (L2) Ethernet network for adding flexibility, scalability and security to the network. VLANs may be defined on different levels, e.g., customer-level or provider-level, and can include any number of non-intersecting domains. Service frame fields preceded with a “C-”, e.g., C-VLAN ID, refers to customer-created fields. Likewise, service frame fields preceded with a “P-” (e.g., P-VLAN ID), refer to provider-added fields. By implementing VLANs, an end-to-end Ethernet network may be partitioned into a number of service instances while preserving multiple subscribers′ C-VLANs, wherein the traffic in a given VLAN is invisible to end hosts belonging to a different VLAN, thus reducing the broadcast domain.
In order to help avoid data loops (which can cause inefficiency due to wasted bandwidth) and recover from failures in an Ethernet network that comprises various bridges in a meshed topology, protocols such as Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP) have been available that allow for dynamic discovery of a topology by the bridges. Essentially, upon transmission of special Configuration Bridge Protocol Data Units (or Configuration BPDUs), a single Root bridge is elected from an Ethernet topology and a Root port is chosen on each bridge that gives the best path from each bridge to the Root. Also, select ports are chosen as Designated Ports for each individual VLAN, which forward frames from the direction of the Root onto a particular VLAN and frames from that VLAN towards the Root. The Root bridge periodically transmits Configuration BPDUs every “hello time,” whereupon the other bridges receiving them transmit a configuration reply message on each of their ports for which they are designated. In case any bridge notices that the STP algorithm has caused it to transition a port into or out of blocking state, it generates a Topology Change Notification (TCN) message towards the Root bridge repeatedly, typically until an acknowledgment is received.
The enhanced functionality of RSTP provides rapid convergence of the spanning tree and allows for fast reconfiguration that is critical for networks carrying delay-sensitive traffic. Additionally, the Multiple Spanning Tree Protocol (MSTP) allows for the mapping of several VLANs to a single spanning-tree instance called MST-Instance (MSTI) wherein each instance is independent of other spanning-tree instances. In this approach, multiple forwarding paths may be provided for data traffic, enabling load balancing. Also, it reduces the number of spanning-tree instances required to support a large number of VLANs.
In the context of Ethernet implementations, e.g., a metro Ethernet, it is typical for a provider network to interconnect multiple customer VLANs. However, it is known that data loops can arise if an attached customer network does not correctly operate its own instance or instances of a spanning-tree, which may degrade the performance of the network to unacceptable levels.